Saturdaynightitis

Step 1: Go out to a pub or nightclub, or stock your house as if it were a drinking/socialising establishment, ie, lots of alcohol and the will to get pissed.
Step 2: Drink the alcohol. The consumption of drugs is optional but not neccessary.
Step 3: Drink more. And more. Repeat steps 2 & 3 until you are capable of step 4.
Step 4: Get to a place to sleep (preferably your own bed) and pass out. Do not clean your teeth, do not drink any water, do not do anything except pass out. If you do anything to wake yourself up, such as drinking coffee, you won't experience Saturdaynightitis.
Step 5: Wake up around 12 hours later. If you've done steps 1 to 4 correctly, you may have slept on your arm, or hand, or leg, on a weird angle. You may be experiencing the feeling of a body part being "asleep"; tingling, kind of painful, uncapable of movement for a period of time decided by how long you lay on the body part undisturbed.

The science that explains Saturdaynightitis

(This explanation uses the hand as an example body part. You can replace 'hand' with arm, leg, foot or any other limb/extremity.)
If you fall asleep with your hand under your body in an unusual position you may exert pressure on the nerves in that hand. Such pressure will squeeze the nerves and stop the messages that travel from your brain to your hand ("Move your hand, you idiot! It hurts!"). As the messages are blocked, so is the oxygen that would usually travel along the paths in the hand. If you're passed out, your brain doesn't get the message to the hand as it normally would if you were awake.

When you wake up after an extended period of sleeping in a weird position your "asleep" body part may take a while to get the blood and oxygen flowing normally again. There's the usual tingling sensation, then the burning pain as the fibres which control pain and temperature get back to normal. Finally, the numbness is the last to go; it took a friend about a day to get full use of her hand back after sleeping on it.